Magnetic recording disks such as floppy disks have been widely employed as external storage devices, with the spread of various office automation apparatuses such as word processors and personal computers. The floppy disk is composed of a flexible non-magnetic support (such as a support of plastic material) and a magnetic layer comprising a ferromagnetic powder such as a ferrite powder.
The magnetic recording disk can be produced by coating a magnetic paint for forming a magnetic layer comprising a ferromagnetic powder, a binder and an organic solvent continuously on a support and drying the coated magnetic paint layer. The coated magnetic layer ought to contain auxiliary materials such as an electroconductive powder, a lubricant and a abrasive powder so as to impart high performance to the magnetic layer, for instance, to increase its abrasion resistance to a practically satisfactory level and/or reduce its surface electric resistance.
The incorporation of the auxiliary materials into the magnetic layer results in decrease of the amount of the magnetic powder, increase of the thickness of the magnetic layer (e.g., 3 to 6 .mu.m), an production of so-called thickness loss such as reduction of output. It is known that the thickness loss can be effectively reduced by decreasing the thickness of the magnetic layer. It is also known that the reduced thickness of the magnetic layer further results in increase of recording density. However, if the thickness of the magnetic layer is reduced, it is difficult to incorporate an enough amount of the auxiliary materials into the magnetic layer. In that case, the surface electric resistance of the magnetic layer is apt to be higher and the endurance decreases. Particularly, if the thickness of the magnetic layer decreases to 2 .mu.m or less, the surface of the magnetic layer is easily influenced by the roughness of the surface of the support whereby the running property deteriorates and the electromagnetic conversion property lowers.
Japanese patent Provisional Publications No. 62-154225 and No. 62-222427 describe a magnetic recording disk having a thinner magnetic layer (0.3 to 1 .mu.m) and a thick non-magnetic undercoating layer which is thicker than the magnetic layer and is provided between the support and the thin magnetic layer. Thus produced magnetic recording disk shows improved electromagnetic conversion characteristics as well as improved running endurance.
Japanese Patent Provisional Publications No. 63-191315, No. H4-325917, No. H5-109061 and No. H6-68453 describe a process for preparing the magnetic recording disk which comprises the steps of: coating on a support a coating dispersion for the non-magnetic layer which contains a non-magnetic powder dispersed in a binder resin solution to form a non-magnetic layer; coating a magnetic paint (i.e., coating dispersion for the magnetic layer which contains a magnetic powder dispersed in a binder resin solution) on the non-magnetic layer while the non-magnetic layer is still wet; and then drying these layers simultaneously. This process enables formation of an extremely thin magnetic layer with little influence of the conditions of the non-magnetic layer. Therefore, the resulting magnetic layer has satisfactory surface smoothness and shows improvement of the electromagnetic conversion property. The above Patent Provisional Publications further describe the incorporation of a lubricant and carbon black (i.e., electroconductive particles) in combination into both layers, i.e., the non-magnetic undercoating layer and the magnetic layer of the magnetic recording disk, so as to further improve the electromagnetic conversion characteristics, anti-static property, and running endurance. Further, description is given for the non-magnetic powder employed for the preparation of the non-magnetic undercoating layer of 0.5 to 5 .mu.m thick.
Requirements for increase of the recording capacity and reduction of the disk size are getting stronger. Therefore, the magnetic recording disk now should ensure, or further improve, high performances on the reproduction output, and running endurance, notwithstanding the requirements for the high recording capacity by the employment of the thinner magnetic layer. However, the thinner magnetic layer such as of not thicker than 0.5 .mu.m likely becomes to lower in its running durability, because the magnetic recording layer of the recording disk is rotated at a high speed in contact with a magnetic head and other members of the disk cartridge. Such rotation of a high speed is necessarily employed to ensure the desired high recording density.
It has been known that incorporation of an aluminum element into a ferromagnetic iron powder is effective to enhance electromagnetic conversion characteristics and running durability. For instance, Japanese Patent Provisional Publication No. H6-215360 describes a magnetic recording medium having a magnetic layer containing a ferromagnetic iron powder which comprises 100 weight parts of an iron atom, 2 to 10 weight parts of an aluminum atom, and 1 to 8 weight parts of a rare earth element atom.